We prove the perfect quantization of the edge spin conductance in spin
Hall insulators . The cancellation of the possible interaction
corrections is particularly remarkable in view of the critical nature
of the problem (M. and Porta, PRB 2017).
Convergence of the expansion holds in a disk with a radius vanishing
with the Fermi velocity and how to overcome this limitation in related
problems will be also discussed (Bonetto and M., AHP 2016).